REMEDIATION OF SUBSURFACE ACID MINE DRAINAGE CONTAMINATED WITH ZN, PB AND CD USING A PERMEABLE REACTIVE BARRIER WITH APATITE II

Phosphate-Induced Metal Stabilization (PIMS) using Apatite II [Ca10-xNax(PO4)6-x(CO3)x(OH)2 where x < 1] stabilizes a wide range of metals, especially Pb, U, Cd, Zn, Cu and Al, in situ or ex situ, by chemically binding them into new phosphate minerals and other low-solubility phases that are stable over geologic time. The excellent stabilization efficiency comes from the extremely low solubility products (Ksp) of the resultant metal-apatites, e.g., for Pb-apatite (pyromorphite) Ksp ~ 10-80 to Ksp ~ 10-167. Combined with this thermodynamic stability, the rapid kinetics of the metal-phosphate precipitation, in the presence of suitable nucleation sites, and of adsorption ensures immobilization of metals in the face of most transport mechanisms. Depending upon the metal, the concentration of the metal and the aqueous chemistry of the system, Apatite II works by four general, non-mutually-exclusive processes: 1) heterogeneous nucleation; 2) pH buffering to 6.5 - 7; 3) non-specific surface chemi-sorption and 4) anaerobic biological stimulation. This technology has been successful with contaminated range soils, groundwaters and wastewaters for Pb, U, Cd, Zn, Al and Cu, and has stabilized between 5% and 50% of its weight in metals depending upon the metal and the environmental conditions. Costs for range soil remediation are $45-$55/yd3 and costs for water remediation are $60 per 1,000,000 gallons of water per mg/L of metal. In this case study, PIMS with Apatite II was used in a subsurface permeable reactive barrier (PRB) to treat acid mine drainage in a shallow alluvial groundwater at Success Mine, Idaho containing elevated concentrations of Zn, Pb, Cd, Cu, SO4 and NO3. Emplaced in 2001, the groundwater is treated in situ before it enters the East Fork of Ninemile Creek, a tributary of the Coeur d'Alene River. The PRB has reduced the concentrations of Cd and Pb to below detection (2 ug/L), has reduced Zn to near background in this region (about 100 ug/L), has reduced SO4 by between 100 and 200 mg/L and NO3 to below detection (50 ug/L). The PRB, filled with 90 metric tons of Apatite II, has removed over 5000 kg of Zn, 100 kg of Pb and 50 kg of Cd, but 90% of the immobilization is occurring in the first 20% of the barrier (the first out of five chambers), wherein the reactive media now contain up to 25 wt% Zn. Field observations indicate that about 30% of the Apatite II material is spent (consumed).